The present invention relates to an image forming apparatus.
In conventional image forming apparatuses of the electrophotographic type such as laser printers, digital copiers, facsimile apparatuses and the like, it is known that a laser beam used for image formation is modulated and output in accordance with image data and reflected by a rotating polyhedral reflective mirror to scan a charged photoconductive member so as to form an electrostatic latent image on the surface of said photoconductive member. The aforesaid polyhedral reflective mirror has a reflective mirror formed so as to have regular polygonal prism-like lateral surfaces as the reflective faces, and is driven in rotation by a motor with the center axis of said polygonal prism as the rotational axis. This type of polyhedral reflective mirror is well known as, for example, a polygonal mirror.
In conventional image forming apparatuses, as shown in FIG. 9, a start-of-scan sensor 42 (hereinafter referred to as "SOS sensor 42") is provided as a photoreceptor sensor at a predetermined position directly in front of the position at which scanning of the photoconductive member starts. In such apparatuses, a laser beam unrelated to image data is forcibly output before scanning (hereinafter referred to as "forced laser beam") to control the output timing of the laser beam used for image formation and output in accordance with image data in the main scanning direction.
The forced laser beam forcibly output from laser diode 23 is reflected by reflective face 33D of polygonal mirror 33, and detected by SOS sensor 42. The output laser beam used for image formation scans the charged surface of the photoconductive member (not illustrated in FIG. 9) in accordance with said detection timing. The image forming laser beam scans the photoconductive member via the next reflective face 33A in accordance with the timing of the forced laser beam detected by SOS sensor 42 in the same manner as for reflective face 33D. The rotational speed of the polygonal mirror is controlled by controlling the voltage applied to drive motor 32 which rotates the polygonal mirror based on the detection of the forced laser beam by each reflective face.
The forced laser beam detection interval of SOS sensor 42, i.e., the time from the moment a forced laser beam reflected by reflective face 33D is detected by SOS sensor 42 until the detection of the next forced laser beam reflected by reflective face 33A (hereinafter referred to as "detection period"), is input to period comparator 112 and compared to a standard period output from standard period generator 111. The voltage applied to drive motor 32 via driver 311 is controlled by a voltage control calculator 113 based on the aforesaid comparison result so as to set the rotational speed of polygonal mirror 32 at a desired speed. That is, the period detected for each reflective face is fed back to adjust the rotational speed of the polygonal mirror 33.
There is concern of dispersion in the width of each reflective face of the polygonal mirror, the angle at which adjacent reflective faces are formed and the distance between each reflective face and the center of rotational axis of the polygonal mirror due to mounting errors of the installed devices and manufacturing errors of the polygonal mirror. When such errors occur, disadvantages arise in the control of the rotational speed of the polygonal mirror.
These disadvantages are described below.
FIG. 10A shows an absence of dispersion of the reflective faces of the polygonal mirror, and FIG. 10B shows a presence of dispersion of the reflective faces of the polygonal mirror. FIG. 10C shows a timing chart of the detection period of SOS sensor 42 in the absence of dispersion of the reflective faces, and FIG. 10D shows a timing chart of the detection period of SOS sensor 42 in the presence of dispersion of the reflective faces. As can be clearly understood from FIG. 10D, the period of detection by SOS sensor 42 in the presence of dispersion of the reflective faces differs from the constant period (standard period T) in the absence of dispersion of the reflective faces (refer to FIG. 10C). When, for example, the width of reflective face 33C is larger than the width of reflective face 33B as shown in FIG. 10B, the period C(K-1) (i.e., the transition from the falling edge B(k-1) to the falling edge C(k-1)) of reflective face 33C becomes [T+.alpha.]. That is, the period of reflective face 33C is longer than the period T of reflective face 33B by the value .alpha.. Similarly, period D(K-1) (i.e., the transition from the falling edge of C(k-1) to the falling edge of D(k-1)) of reflective face 33D is [t+.beta.], and is longer than period T of reflective face 33B by the value .beta..
FIGS. 11A through 11C show the control of the rotational speed of the polygonal mirror in the absence of dispersion of the reflective faces, and FIGS. 11D and 11E show the control of the rotational speed of the polygonal mirror in the presence of dispersion of the reflective faces.
As shown in FIGS. 11A through 11C, when the rotational speed of polygonal mirror 33 is changed in the absence of dispersion of the reflective faces, e.g., when the period of reflective face 33A becomes [t+.alpha.], the rotational speed of the next reflective face 33B is adjusted so as to increase said speed by the value .alpha.. As a result, the period of reflective face 33B becomes [t]. Similarly, when the period of reflective face 33A becomes [t-.alpha.], the rotational speed of reflective face 33B is adjusted so as to decrease said speed by the value .alpha., such that the period of reflective face 33B becomes [t]. In other words, in the absence of dispersion, the rotational speed of polygonal mirror 33 is adjusted to a desired speed by the aforesaid controls.
On the other hand, as shown in FIG. 11D and 11E, when the rotational speed of polygonal mirror 33 is not changed in the presence of dispersion of the reflective faces, e.g., when the period of reflective face 33A is [t+.alpha.], the aforesaid controls adjust the rotational speed of the next reflective face 33B so as to increase said speed by the value .alpha.. As a result, the period of reflective face 33B is shorter than the previous period by the value .alpha.. In the presence of dispersion of reflective face 33B, since the original period of reflective face 33B (i.e., the period of reflective face 33B when the rotational speed is adjusted to a desired speed) is [T+2.alpha.], when the aforesaid control executed, the speed is adjusted so as to increase said speed by the value .alpha., such that the period of reflective face 33B becomes [T-3.alpha.]. That is, when the aforesaid control is executed in the presence of dispersion of the reflective face, the rotational speed of polygonal mirror 33 differs markedly from the desired speed.